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Comment by keredson | Hacker News Reader

On the dowel there are 5 steel nuts which are pulled towards the electromagnet. Gravity makes them return, but TBH it's not reliable enough. I'm working on a new version with small return springs and a wider swing for the striker.

The electromagnet is pulsed to control how hard to strike.

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Really impressive work!

I am really interested in what you are able to come up with.

I remember taking apart old telephones with bell ringing mechanisms.

That might be a source of further design inspiration for you.

Maybe ballpoint pen springs?

I purchased a child’s xylophone to try something similar with, but the cost of the solenoids was a dealbreaker for me at the time ($6+ per solenoid on eBay).

Hah, yeah, I think I saw them on Amazon for like $20. That seemed nuts; why I decided to roll my own.

Even more crazy were these "iron core for electromagnets" https://www.amazon.com/Soft-Ideal-making-electromagnets-inch..., 6" for $17.

Give me a break. Soft steel is >99% iron and I bought 4' for <1/2 that price from HD. Rip-off.

Did you consider using servos vs electromagnets? Curious if small servos couldn’t accomplish the same goal.

I hadn't even thought of using small servos. Possibly that would have been a lot simpler! I went though a bunch of prototype EMs testing to get a working combo of voltage, windings, core size, etc. (Blew more than a few driver circuits too...)

That is a good idea. Those 9g mini servos are only $2 each, so that is much more affordable than the ‘retail’ solenoids.

If you are willing to wait on them, you can get 5 packs of 9g servos for $2 including shipping from China.

I am interested in that url, please.

I’m not seeing them in stock with the sellers I usually buy from, but here is a 4 pack for $4.39 including shipping from China ($2.09 from US, but not free shipping):

https://m.aliexpress.com/item/4000066002225.html

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